1. Field of the Invention
This invention relates to an apparatus for determining the concentration of a chemical substance in a gas. More particularly, the invention relates to a system determining the concentration of hydrogen in a gas by adsorbing hydrogen onto a light-carrying optic fiber and measuring the change in light transmitted through the fiber.
2. Discussion of Background
Apparatus for detecting and measuring the concentration of specific chemicals in gases have various applications, including, detection of toxic or explosive gases in the atmosphere, monitoring the constituents in industrial process streams and detection of contaminants in ground water. There are many different apparatus for such purposes, responding to electrical, chemical or optical effects induced by the presence of the chemical substance in the carrying fluid.
Each type of detector has characteristics that are advantageous for some uses and inappropriate for others. Detectors which depend on electrical effects may be influenced adversely by electromagnetic interference. Electronic detectors can also be hazardous if used for the detection of substances in flammable or explosive gases because of the risk of ignition.
Another method for detecting chemical constituents, gas chromatography, is somewhat time consuming and costly, requiring elaborate handling procedures for the sample and trained technicians to operating the gas chromatograph.
Some types of detectors can become contaminated by other constituents that interfere with the reactive portion of the sensor preventing accurate measurements. Most detectors are poisoned by nitric oxide, for example, a gas generated as a by-product of many of the processes which generate hydrogen. In many cases this pollutant results in complete failure of the detector.
Another disadvantage of the existing detectors is limited monitoring capacity. Usually, one detector or sensor monitors one location of a process stream. Multiple sensors are therefore needed to monitor the complete system, adding greatly to the cost of operation. Even with the advent of "smart sensors," silicon chips where all the components are incorporated into a single, inexpensive chip, disadvantages in the multiple sensor requirement remain. While size and cost may be reduced, these sensors are not impervious to poisons such as nitric oxide and service time may be short.
A particular type of detector is disclosed in U.S. Pat. No. 4,834,497 issued to Angel. In Angel's device, chemical substances are detected when they are absorbed onto the coating or cladding of a fiber optic element. To absorb, of course, means to swallow up, to include or take a thing in to the loss of its separate existence, to incorporate. To adsorb means to form a compound upon the adherence of specific liquids or gases in solution to surfaces of materials, usually solids, with which they are in contact. Angel's device is based on the former, absorption.
Several industrial processes generate hydrogen, an explosive gas. Specific limits on the concentration of hydrogen are imposed by law and sound industrial practice to assure occupational safety. Therefore, monitoring of hydrogen concentrations is both appropriate and mandatory.
It is known that hydrogen is weakly adsorbed onto the silica of optical fibers and that this adsorption adversely affects the transmission of data in communication networks. Gaseous molecular hydrogen (H.sub.2) itself does not adsorb infrared light, but molecular hydrogen weakly adsorbed on the silica of an optical fiber becomes polarized and actively absorbs infrared radiation. H.sub.2, according to the literature, adsorbed on SiO.sub.2 forms a weak Si--H bond which has a fundamental vibration of 4.6 micrometers. The absorption of light at 2.42 micrometers is caused by an overtone of the fundamental. The frequency of the absorption band (4132 cm.sup.-1) is very close to the fundamental vibration (4160 cm.sup.-1) of gaseous hydrogen. A buffer is commonly applied to the exterior of optical fibers used in the transmission of data and telecommunications to reduce the adsorption of hydrogen and increase the long term use of optical fibers. Without the buffer, the optical fiber adsorbs hydrogen and the increased absorption affects the quality of the signal. A description of this effect is found in "Behavior of Hydrogen Molecules of Silica in Optical Fibers", Mochizuki, et al., IEEE Journal of Quantum Electronics, Vol. QE-20, No. 7, July 1984. It is believed this effect has not heretofore been used for hydrogen detection.
The present invention is directed to overcoming one or more of the problems related to the detection of hydrogen or other substances in a gas.